Via a German blog I was attended to this site/device and I was immediately sold and wanted to have one. But for a while there's no money left. So I thought of trying to build my own.
To test if that is possible in the first place I first did a very quick and dirty attempt and here's it is: A bread board with 3 turnable closet wheels. All together about €10,-
And the result: look here for 2 foto's and a first clip. This clip was made by just gently pushing the bread board. It curves by itself because of the position of the 2 upper wheels. The radius of the curve is determined by the angle of the wheels. I myself was amazed by the result.